In the drilling and completion industry, the formation of boreholes for the purpose of production or injection of fluid is common. Hydrocarbons such as oil and gas can be recovered from the subterranean formation using the boreholes. Various operations require the pumping of fluid into the borehole. In many instances, it is necessary to pump a large volume of fluid into the borehole. For example, hydraulic fracture stimulation operations often require the concurrent use of multiple fracturing fluid pumping units at a single well site in order to provide the desired quantity of fracturing fluid needed to fracture the earthen formation. Typically, multiple trailer or skid mounted hydraulic fracturing fluid pumping units, each including a single diesel motor, driveline and a single pump, are simultaneously used to provide the requisite demand of fracturing fluid into the borehole.
While the use of an electric motor in place of a diesel motor could reduce weight on the skid and create less undesirable exhaust emissions at the well site, large horsepower electric drives create large inrush starting currents (the maximum, instantaneous input current drawn by an electrical device when first turned on). The use of high capacity distribution wire and/or sub-station transformers forces higher watt-hour (“Wh”) utility rates and other associated costs. The normal operating power of large electric driven pumps and compressors is approximately 0.15-0.25 of locked rotor start inrush. Mitigation schemes include variable frequency drive (“VFD”) controls, soft-start devices, and reduced voltage operation. However, all of these starting methods are problematic in the harsh oilfield environment, with respect to one or more of size, weight, complexity, and cost.
Natural gas has also been employed to drive a dedicated on-site turbine generator to eliminate the need for a transmission in the production of electricity, to power the fracturing modules, blenders, and other on-site operations as necessary, including other local equipment, including coiled tubing systems and service rigs. The use of a dedicated power source has been preferred over grid power because during startup of a fracturing operation, massive amounts of power are required such that the use of grid power would be impractical. The potential for very large instantaneous adjustments in power drawn from the grid during a fracturing operation could jeopardize the stability and reliability of the grid power system, as well as result in increased costs passed on to the operator. Accordingly, a site-generated and dedicated source of electricity has provided a more feasible solution in powering an electric fracturing system. While providing an alternative to grid powered systems, the use of site-generated sources of electricity necessitates extra equipment at the well site.
The art would be receptive to alternative devices and methods useful in connection with enabling the use of electric motors in downhole fluid delivery operations without incurring the above-described problems.